Variable-Angle 3-Dimensional Exchange Nuclear-Magnetic-Resonance Spectroscopy for the Study of Molecular-Motion in Complex Solids

Although molecular motions are responsible for many of the macroscopic properties observed in solids, especially in polymers, methods for studying these processes in all but the simplest systems are scarce. In the present study we introduce a three-dimensional nuclear magnetic resonance experiment for characterizing ultraslow molecular motions in complex solid systems. The technique extracts dynamic information by resolving the two-dimensional exchange distributions that can be observed in spectra of static samples, according to the isotropic chemical shifts of individual molecular sites. These three-dimensional correlations are achieved by processing signals arising from a fast-spinning solid sample using two independent macroscopic axes of rotation as extraction parameters, an approach which becomes practical due to the simple scaling behavior of anisotropic chemical shifts with respect to the axis of sample rotation. The principles involved in this new spectroscopic technique are discussed, and the method is illustrated with an application to the analysis of motions in isotactic polypropylene.